This invention relates to electret tapes and more particularly to improved apparatus for and method of making such tapes.
U.S. Pat. No. 4,213,168 describes apparatus for making an electret tape in which the conductive side of the tape engages and is moved longitudinally by a driver capstan while the Teflon covered tape side passes under a fluid saturated sponge connected to a charging voltage. An air blower dries the tape surface as it exits from the charging sponge. One problem with this technique is the tendency of the sponge to dry out at high tape speeds. This drying problem is exacerbated by the blower air stream that is deflected by the tape onto the sponge. A dry sponge fails in whole or in part to electrostatically charge the Teflon layer resulting in a defective electret tape. Another problem experienced at high production tape speeds and with the steep drop-off angles of the captan surface that droplets of charging fluid separate from the sponge which results in non-uniform charging of the Teflon layer. The reason for this is electrostatic charges recombine under droplets that have left the electric field, causing dead spots or voids in the electret. Also, opposing forces of the forwardly moving tape and backward air flow can cause the separated droplet to remain stationary on the Teflon layer, creating an uncharged line or stripe.
The two problems described above are especially troublesome because compensation of one tends to aggravate the other. Increasing fluid supply to the sponge to compensate the drying effect increases formation of break-away droplets. Increasing the velocity of air flow to dry the fluid more rapidly to impede droplet break-away causes the sponge to dry out, a condition worse than an overly wet sponge.
This invention is directed to a solution of these problems.